(a). Field of the Invention
This invention relates to a sintered liquid phase stainless steel with enhanced machinability characteristics. Essentially full density austenitic stainless steel parts are made starting from a powdered composition known from the prior art to which manganese sulfide powder has been added.
(b). Description of the Prior Art
Reference is made to prior U.S. Pat. Nos. 3,980,444 (Reen), 4,014,680 (Reen), and 4,032,336 (Reen). These patents disclose a sintered stainless steel having an overall density of at least 95% of full density and a morphology comprised of regions of sintered austenitic stainless steel and regions of solidified liquid phase. Conventional sintering is a process of metallurgically bonding metallic powder without melting. Generally speaking, the powder is first pressed into the desired part and then heated in an oven. The cited patents teach a sintered stainless steel having a composition consisting essentially of, by weight, up to 0.05% carbon, 22% to 26% chromium, 10% to 24% nickel, 2.7% to 5% molybdenum, 0.1% to 1% boron, up to 2.0% manganese, up to 2.0% silicon, and the balance iron and residuals. U.S. Pat. No. 4,014,680 discloses this composition as a pre-alloyed stainless steel powder to be used in the sintering process. U.S. Pat. No. 3,980,444 discloses the sintered stainless steel which results from this composition, and U.S. Pat. No. 4,032,336 discloses the method of making sintered stainless steel having this composition by using the said stainless steel powder. The patents further disclose that the use of the composition increases the density of the sintered stainless steel, resulting in increased resistance to corrosive attack by the chloride ion.
The composition and methods disclosed in the previous patents have proven their worth over many years of use. Reference may be made to American Society for Testing Materials Standard Specification ASTM B 853 covering stainless steel powder metallurgy structural components fabricated from pre-alloyed powder consisting primarily of iron, chromium, nickel, molybdenum and boron which are intended for use in corrosive service. The composition has also been given designation S31905 by the Unified Numbering System for Metals and Alloys.
Solid parts made according to the composition and process described in the three patents achieve high density by forming a liquid phase during the sintering process, known as liquid phase sintering. After the part has been liquid phase sintered, however, it is often still necessary to machine it. For example, the part may need to be machined to obtain a specific dimensional tolerance, or may require threading--something which cannot be done during the pressing process. Unfortunately, the material is difficult to machine. In this respect the sintered stainless steel parts according to the three previous patents have an austenitic microstructure similar to austenitic stainless steels in wrought form, such as bar stock, made by conventional steel-making processes using melting. Machinability shortcomings with wrought steel have been alleviated by introducing specific particles, both metallic and non-metallic, throughout the stainless steel matrix. These particles historically have been lead, selenium sulfide, and manganese sulfide. Because of toxicity concerns in the melting process, the production and use of wrought stainless steel containing lead and selenium sulfide have been declining, leaving manganese sulfide as the generally used material in the industry for improving the machinability of wrought stainless steel. In the normal melting practice of producing wrought stainless steel, manganese sulfide particles are produced by stoichiometric additions of manganese and sulfur for manganese sulfide formation. The manganese content of the molten metal for atomizing powder must be kept to a maximum of 0.15%. This would preclude the use of wrought austenitic stainless steel for remelt stock due to its high manganese content. If higher values of manganese are used in the melt, the resultant atomized powder is spherical in shape. Spherically shaped powders are very undesirable for making pressed and sintered parts because the parts may have little or no green strength. Therefore, manganese sulfide particles cannot be produced in the matrix of the atomized stainless steel powder by conventional melting procedures.